This invention relates to a method of controlling the idling rotational speed of an internal combustion engine for vehicles equipped with air conditioning systems.
A compressor in an air conditioning system for vehicles is generally disposed to be driven by an internal combustion engine installed on the vehicle. The rotational speed of the engine is controlled to increase upon the start of the compressor during idling of the engine (so-called idling-up), in order to prevent a drop in the engine rotational speed due to increased load applied on the engine by the compressor.
According to conventional methods, the amount of a control parameter, e.g. intake air or fuel injection amount, is increased by a fixed amount to increase the idling enigne rotational speed irrespective of the magnitude of torque required for starting the compressor and applied on the engine as load. However, the magnitude of the starting torque of the compressor required under a condition of high ambient temperature and high thermal load on the compressor is quite different from that required under a condition of low ambient temperature and low thermal load on same, so that the idling engine rotational speed abruptly increases under condition of low ambient temperature and low thermal load at which the starting torque of the compressor is relatively small.
In order to overcome the above disadvantage, an air conditioning system has been proposed by Japanese Patent Publication (Kokoku) No. 62-47732, in which the idling engine rotational speed is controlled depending upon the capacity at which a variable capacity compressor thereof is operating, and which determines the starting torque of the compressor applied on the engine as load, so as to prevent an abrupt increase in the idling engine rotational speed when the compressor is started at low capacity and low thermal load.
However, according to the proposed air conditioning system, it is necessary to detect the actual capacity of the compressor to control the idling engine rotational speed, which requires the employment of a sensor for sensing the actual capacity of the compressor. Such a sensor causes an increased cost. Further, the compressor with the sensor mounted therein will be complicated in construction and large in size.